Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Solar cells are typically manufactured using the same processes used for other semiconductor devices, often using silicon as the substrate material. A semiconductor solar cell is a simple device having an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric field is typically created through the formation of a p-n junction (diode), which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity.
FIG. 3 shows a first embodiment of a solar cell, and is a cross section of a representative substrate 300. Photons 301 enter the solar cell 300 through the top surface 305, as signified by the arrows. These photons pass through an anti-reflective coating 310, designed to maximize the number of photons that penetrate the substrate 300 and minimize those that are reflected away from the substrate.
Internally, the substrate 300 is formed so as to have a p-n junction 320. This junction is shown as being substantially parallel to the top surface 305 of the substrate 300 although there are other implementations where the junction may not be parallel to the surface. The solar cell is fabricated such that the photons enter the substrate through a heavily doped region, also known as the emitter 330. In some embodiments, the emitter 330 may be an n-type doped region, while in other embodiments, the emitter may be a p-type doped region. The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction. Thus any e-h pairs that are generated in the depletion region of the p-n junction get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons are absorbed in near surface regions of the device, the minority carriers generated in the emitter need to diffuse across the depth of the emitter to reach the depletion region and get swept across to the other side. Thus to maximize the collection of photo-generated current and minimize the chances of carrier recombination in the emitter, it is preferable to have the emitter region 330 be very shallow.
Some photons pass through the emitter region 330 and enter the base 340. In the scenario where the emitter 330 is an n-type region, the base 340 is a p-type doped region. These photons can then excite electrons within the base 340, which are free to move into the emitter region 330, while the associated holes remain in the base 340. Alternatively, in the case where the emitter 330 is a p-type doped region, the base is an n-type doped region. In this case, these photons can then excite electrons within the base 340, which remain in the base region 340, while the associated holes move into the emitter 330. As a result of the charge separation caused by the presence of this p-n junction, the extra carriers (electrons and holes) generated by the photons can then be used to drive an external load to complete the circuit.
By externally connecting the emitter region 330 to the base 340 through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts 350, typically metallic, are placed on the outer surface of the emitter region and the base. Since the base does not receive the photons directly, typically its contact 350b is placed along the entire outer surface. In contrast, the outer surface of the emitter region receives photons and therefore cannot be completely covered with contacts. However, if the electrons have to travel great distances to the contact, the series resistance of the cell increases, which lowers the power output. In an attempt to balance these two considerations (the distance that the free electrons must travel to the contact, and the amount of exposed emitter surface 360), most applications use contacts 350a that are in the form of fingers. FIG. 4 shows a top view of the solar cell of FIG. 3. The contacts are typically formed so as to be relatively thin, while extending the width of the solar cell. In this way, free electrons need not travel great distances, but much of the outer surface of the emitter is exposed to the photons. Typical contact fingers 350a on the front side of the substrate are 0.1 mm with an accuracy of 41-0.1 mm. These fingers 350a are typically spaced between 1-5 mm apart from one another. While these dimensions are typical, other dimensions are possible and contemplated herein.
A further enhancement to solar cells is the addition of heavily doped substrate contact regions. FIG. 5 shows a cross section of this enhanced solar cell. The cell is as described above in connection with FIG. 3, but includes heavily doped contact regions 370. These heavily doped contact regions 370 correspond to the areas where the metallic fingers 350a will be affixed to the substrate 300. The introduction of these heavily doped contact regions 370 allows much better contact between the substrate 300 and the metallic fingers 350a and significantly lowers the series resistance of the cell. This pattern of including heavily doped regions on the surface of the substrate is commonly referred to as selective emitter design.
A selective emitter design for a solar cell also has the advantage of higher efficiency cells due to reduced minority carrier losses through recombination due to lower dopant/impurity dose in the exposed regions of the emitter layer. The higher doping under the contact regions provides a field that repels the minority carriers generated in the emitter and pushes them towards the p-n junction.
The embodiment shown in FIG. 3 requires contacts on both sides of the substrate, thereby reducing the available area of the front surface through which photons may pass. A cross section of a second embodiment of a solar cell 400 is shown in FIG. 6. Fundamentally, the physics of this embodiment is similar, in which a p-n junction is used to create an electric field which separates the generated electron hole pairs. However, rather than create the p-n junction across the entire substrate, as done in the previous embodiment, the junctions are only created in portions of the substrate 400. In this embodiment, a negatively doped silicon substrate 410 may be used. In certain embodiments, a more negatively biased front surface field (FSF) 420 is created by implanting addition n-type dopants in the front surface. This front surface is then coated with an anti-reflective material 430. This front surface is often etched to create a sawtooth or other non-planar surface, so as to increase surface area. The metallic contacts or fingers 470 are all located on the bottom surface of the substrate. Certain portions of the bottom surface may be implanted with p-type dopants to create emitters 440. Other portions are implanted with n-type dopants to create more negatively biased back surface field 450. The back surface is coated with a dielectric layer 460 to enhance the reflectivity of the back surface. Metal fingers 470a are attached to the emitter 440 and fingers 470b attaches to the BSF 450. FIG. 7 shows one commonly used configuration of the metal fingers on the back surface. This type of cell is known as an interdigitated back contact (IBC) solar cell.
With current energy costs and environmental concerns, solar cells are becoming increasingly important. Any reduced cost to the manufacturing or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Current solar cell design is limited by the dopant profiles that can be achieved by diffusing dopants into the silicon of the solar cell. It is also limited by the difficulty in applying different dopants and doping different parts of a solar cell. The cost of additional doping steps limits the solar cell designs that may be used commercially. Accordingly, there is a need in the art for improved ion implantation of solar cells and, more particularly, a method that allows successive doping steps of a solar cell with an ion implanter.